1. Field of the Invention
The present invention relates to an exhaust gas NO.sub.x removal system for removing nitrogen oxides contained in exhaust gas, and more particularly to a NO.sub.x removal system that is well-suited for removing nitrogen oxides contained in exhaust gas discharged from a gas turbine, a diesel engine, a gas engine, a boiler, a heating furnace, a refuse incinerator, an FCC, a chemical reaction process, etc.
2. Description of the Prior Art
An exhaust gas NO.sub.x removal system in the prior art for cleaning combustion exhaust gas discharged from a gas turbine of a gas turbine power generating system will be described with reference to FIGS. 2 and 3. As shown in these figures, fuel 2 and air 3 are charged into a gas turbine 1 and combusted, and the combustion exhaust gas is sent to a flue 4. The exhaust gas in the flue passes through a heat-exchanger 5 for recovering heat of the exhaust gas, which is used to generate steam, and then through an NO.sub.x removal system 6 provided with an ammonia injector. Afterwards, the exhaust gas is dispersed into the atmosphere through a stack 7. The above-mentioned heat-exchanger 5 constitutes a vaporizer, a superheater and the like of an exhaust gas boiler making use of the exhaust gas of the gas turbine 1 as a heat source.
The NO.sub.x removal system 6 provided in the flue 4 is for reducing nitrogen oxides (hereinafter abbreviated as NO.sub.x) in the combustion exhaust gas into harmless nitrogen and water through a catalyst by injecting ammonia.
In one known type of the NO.sub.x removal system 6 illustrated in FIG. 2, after aqueous ammonia 8 is evaporated by NO.sub.x -free exhaust gas extracted from the flue 4, it is sprayed into the above-mentioned NO.sub.x removal system 6. The NO.sub.x -free exhaust gas is extracted from the flue by an exhaust gas recirculation fan 10, and is then introduced to a vaporizer 11. Here, aqueous ammonia is atomized with air or steam 9, and a gaseous mixture of aqueous ammonia and the exhaust gas extracted by the exhaust gas recirculation fan 10, may be injected into the flue 4 upstream of the catalyst layer of the NO.sub.x removal system 6 via an ammonia vapor pipe 12.
In addition, one example of a system in which aqueous ammonia is directly sprayed into an NO.sub.x removal system 6 is shown in FIG. 3. In this figure, spray nozzles 10a are disposed upstream of a catalyst layer in the NO.sub.x removal system 6 disposed within a flue 4. Aqueous ammonia stored in an aqueous ammonia tank 13 is sent under pressure to the spray nozzles 10a by means of a pump 14, whereby the ammonia is sprayed onto the catalyst layer. It is to be noted the reference numeral 15 designates a mixer, and the other elements similar to those shown in FIG. 2 are designated by like reference numerals.
In the case where aqueous ammonia ( or its precursor, for instance, aqueous urea) is vaporized by NO.sub.x -free exhaust gas and is injected into the flue, since sulfur oxides or a minute amount of nitrogen oxides in the exhaust gas are combined with the ammonia, ammonium sulfate, ammonium acid sulfate or ammonium nitrate in a solid state are produced. Solids of these products would adhere to an orifice by which a flow rate is measured or to a damper provided in the ammonia vapor pipe. To prevent such a problem it is necessary to maintain a high temperature at the outlet of the vaporizer, that is, to sustain a vapor temperature of aqueous ammonia at 200.degree. C. or higher. As a result, it was necessary to extract a large amount of exhaust gas.
In the case of directly dispersing aqueous ammonia into the exhaust gas as shown in FIG. 3, the rate at which the vaporization reaction of liquid occurs is a controlling factor. Because such a reaction takes as long as 0.5-1.0 second, it was necessary to space injection nozzles a large distance from an NO.sub.x removal catalyst, to form a fine spray of ammonia or to provide a mixer for preliminarily mixing the aqueous ammonia with exhaust gas.